No, no, no! This mapping isn't even using gnu grep as it claims, it's using the :grep Vim command (which, on a Unix system, will utilize the grep tool by default, but it is not at all the same). If you want to limit the file type, just pass in the desired files into the command itself! There is no need to shell out to a find command! See Find in files within Vim.

TO DO
Merge this tip somewhere. It basically consists of an incorrect description, and a few very simple mappings.